1. Field of the Invention
The present invention relates to a color stencil printer, and more particularly to an improvement in printing quality resulting from a difference in the dot gain of ink, particularly a variation in hue.
2. Description of the Related Art
A printer, such as a thermal printer, a copying machine, a stencil printer and the like are known as an image forming apparatus which outputs the same image as a read image to printing paper or the like, based on an original image signal read by a reading section having a charge-coupled device (CCD) line sensor or the like.
In the above-mentioned stencil printer, a stencil making section, consisting of a thermal head and a platen roller, perforates a stencil paper, based on an original image signal read by the reading section. Then, the perforated stencil paper is wound around a printing drum. Next, printing paper is inserted between the printing drum and a press roller which rotates in contact with the printing drum. Ink inside the printing drum is pushed out to the printing paper through the perforations in the perforated stencil paper, whereby ink transfer is performed. In this way, printing is performed.
With recent advances in image coloring, color stencil printers have been proposed which make a perforated stencil paper using a thermal head for each color to be used in printing and perform color printing by performing overprinting, using the perforated stencil paper for each color.
When color printing is performed with the color stencil printer, enhancing image gradation or printing in a desired tone of color is very important in order to enhance the quality of a printed image. For that purpose, gradation and color-tone controls are indispensable. To perform these controls, scanner xcex3-correction, color-tone control, printer xcex3-correction, gradation control and the like have hitherto been performed.
As a method of embodying gradation control in this color processing, a method of controlling printing for each color by various kinds of image processing, such as half tone processing (such as a dithering method and an error diffusing method) and the like, is common. Also, as a method of embodying color-tone control, it is common to convert a read color image to an arbitrary color space and then adjust the image so that it has a desired tone of color on the color space.
In such methods of embodying gradation control and color-tone control, control is performed on the assumption that the dot gains of inks to be used in printing are all the same. The xe2x80x9cdot gainxe2x80x9d used herein means a ratio of the size of ink, transferred onto a printing paper through a perforation formed in a stencil paper, to the size of the perforation.
The control is also performed on the assumption that ink to be used is ideal ink in which the spectral density, as shown in FIG. 7, includes only the spectral component of a necessary color for each color and does not include an unnecessary spectral component. Furthermore, the control is performed on the assumption that, if inks differ in ink type but are the same in color, the spectral densities are also the same.
However, the dot gains of inks that are actually used are not always the same. If inks differ in color, the dot gains thereof will differ, and even if inks are the same in color but if inks differ in ink type, the dot gains will differ. For this reason, if printing is performed with inks differing in dot gain by a single thermal head, there is a problem that desired printing quality will not be obtained, even if the above-mentioned gradation or color-tone control were performed. For instance, a magenta color becomes lighter as a whole than a cyan color. If overprinting is performed, a bluish image will be obtained, or it will be reversed. Thus, a desired tone of color cannot be brought out.
As primary factors having influence on the dot gain of ink, the following are considered. What is expressed to be greater by a sign of inequality is that the dot gain of ink is greater.
(1) Ink, a Master Fiber, and Paper
Ink viscosity: soft greater than hard
Master fiber: Japanese paper greater than chemical fiber
Surface roughness of paper: RISO paper greater than fine paper (RISO SR paper) greater than simple coated paper (inkjet paper) greater than rough dyed paper (smoother than inkjet paper) greater than perfect coated paper
The xe2x80x9cRISO paperxe2x80x9d and xe2x80x9cRISO SR paperxe2x80x9d here means printing paper made by RISO KAGAKU Corporation.
(2) Control Information
Platen pressure: strong greater than weak
Pressure-applying time: long greater than short
Printing speed: slow greater than fast
(3) Others
Temperature: high greater than low
In addition, the spectral density of each ink to be actually used includes not only the spectral component of a necessary color but also unnecessary spectral absorption called auxiliary absorption. For example, yellow ink absorbs not only a Blue component but also components other than the Blue component. Therefore, each ink that is actually used has spectral density such as that shown in FIG. 8. For that reason, in mixing colors, the inks will become too deep because a certain spectral density is polarized. Therefore, even if the dot gains of all inks are the same, a problem that desired printing quality (tone of color) will not be obtained will arise. Furthermore, there are cases where even if inks are the same color, the spectral densities will differ if the inks differ in ink type. For that reason, even if perforation sizes are the same, a problem will arise that the spectral densities of inks will differ if the inks differ in ink type.
In view of the foregoing observations and description, the primary object of the present invention is to provide a color stencil printer which is capable of obtaining desired printing quality even when the dot gains of inks to be used in printing differ or even when the spectral densities of the inks are not ideal.
To achieve the above object and in accordance with the present invention, there is provided a color stencil printer comprising
stencil making means which perforates a heat-sensitive stencil paper using a thermal head, based on input binary image data, and
printing means which performs printing by transferring ink to printing paper through a perforation of the perforated stencil paper,
wherein the stencil making means includes
dot-gain information input means which inputs dot-gain information representative of a dot gain of ink for each ink type to be used in the printing and/or spectral-density information input means which inputs spectral-density information representative of the spectral density of the ink, and
perforation-size control means which controls the size of the perforation, based on the dot-gain information and/or the spectral-density information.
In a preferred form of the present invention, the perforation-size control means performs gradation control and/or color-tone control by controlling the size of the perforation.
In the color stencil printer according to the present invention, the binary image data may be data on which half tone processing has been performed.
According to the stencil printer of the present invention, the size of the perforation is controlled for each ink type to be used in printing, based on the dot gain and spectral density of the ink. Therefore, even if the dot gains of inks differ, or even if the spectral densities of the inks are not ideal, the perforation can be formed into a suitable printing dot size in consideration of the dot gain and spectral density of ink to be actually used.
In addition, the perforation size is controlled to control printing dot size. By controlling this perforation size, it becomes possible to perform gradation control and/or color-tone control. Although the color processing in the conventional stencil printer requires image processing such as scanner xcex3-correction, color-tone control, printer xcex3-correction, gradation processing and the like, the stencil printer of the present invention renders simple and free color adjustments possible. Furthermore, since the perforation size is controlled for each ink type in consideration of the dot gain and spectral density of the ink, printing can always be performed in a predetermined printing dot size, even if inks differ in ink type.